Mix together one-half pound of soda, one-half pound of soft soap and
one pound of whiting. Boil them until they become as thick as paste,
and let it cool. Before it is quite cold, spread it over the surface
of the marble and leave it at least a whole day. Use soft water to
wash it off, and rub it well with soft cloths. For a black marble,
nothing it better than spirits of turpentine.
Another paste answers the same purpose: Take two parts of soda, one of
pumice stone and one of finely-powdered chalk. Sift these through a
fine sieve and mix them into a paste with water. Rub this well all
over the marble and the stains will be removed; then wash it with soap
and water and a beautiful bright polish will be produced.